By my calculations, mining for Bitcoins on my GTX 560 Ti is still profitable... but margins are razor-thin, and with the next difficulty level, there won't be a point, since all Bitcoins will be earned at a slight loss due to the increase in my electric bill.

It'll probably happen within one or two years for those that have AMD cards and significantly more efficient setups. You hit a wall, and realize that you are paying more for electricity than you can earn in Bitcoins.

Won't most miners leave, causing no more Bitcoins to enter the system?

That too, I suppose. But I'm not going to sell my GTX 560 Ti and swap it for a 6870 just for the sake of "mining" a cryptographic commodity that may have little use in the future. There is more speculation around Bitcoin than there is commerce conducted with it, right?

It's unlikely to ever become unprofitable though it may become somewhat less profitable. Mining has an associated cost, unless you live in a dorm or with your parents - and if you're mining at any real scale in a dorm or parents' basement, someone is going to come yelling at you about the electric bill REALLY soon...

If most people can, after cost of operations, make $1,000 per month from a small mining operation, that's cool and they're likely to keep doing it. They might even buy new hardware to expand their operation, thus increasing the total hashrate of the network and driving difficulty up - therefore driving profit down.

If, after cost of operations, they can make $100 per month from that same operation, some will keep mining, but are highly unlikely to expand, and many will decide that the time, electricity, air conditioning, noise and heat are simply no longer worth it for a mere $100. Now the hashrate decreases and difficulty goes down.

Difficulty is similar to price in that they are both trying to find some stable equilibrium. If mining ever does dip into "unprofitable" territory it will likely only do so for a very brief time, similar to the value of BTC after the $31 bubble burst and it found its way back to equilibrium. Realistically there will be a level of "less profitable" that miners will begin to leave at, thus balancing around a particular difficulty/price ratio.

Homeostasis applies to more things than the populous at large seems to realize, and it applies here too.